The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) apparatus and method for providing energy to biological tissue, and more particularly, to a catheter having an electrode with a non-joined thermocouple for providing multiple temperature-sensitive junctions on the electrode.
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C.
During ablation, portions of the electrodes are typically in contact with the blood, so that it is possible for clotting and boiling of blood to occur if those electrodes reach an excessive temperature. Both of these conditions are undesirable. Clotting is particularly troublesome at the surface of the catheter electrode because the impedance at the electrode rises to a level where the power delivery is insufficient to effect ablation. The catheter must be removed and cleaned before the procedure can continue. Additionally, too great a rise in impedance can result in sparking and thrombus formation within the heart, both of which are also undesirable.
Further, too great a temperature at the interface between the electrode and the tissue can cause the tissue to reach a high impedance which will attenuate and even block the further transmission of RF energy into the tissue thereby interfering with ablation of tissue at that location.
Even though no significant amount of heat is generated in the electrodes themselves, adjacent heated endocardial tissue heats the electrodes via heat conduction through the tissue. As mentioned above, part of the active electrode will be in contact with the blood in the heart and if the electrode temperature exceeds 90-100xc2x0, it can result in blood boiling and clotting on the electrode. The application of RF energy must then be stopped. However, shutting the RF generator off due to the temperature rise may not allow sufficient time to complete the entire ablation procedure. Providing an ablation electrode capable of applying higher amounts of power for a longer period of time to ablate the damaged tissue to an acceptable depth is a goal of current ablation catheter electrode design. It has been found that higher power for longer time periods results in a higher probability of success of the ablation procedure.
To avoid clotting and blood boiling, RF ablation catheters for cardiac applications typically provide temperature feedback during ablation via a temperature sensor such as a thermocouple. In its simplest form, a thermocouple consists of two dissimilar metals joined together at one end called a xe2x80x9cbeadxe2x80x9d or junction, such as a conventional copper/constantan type xe2x80x9cTxe2x80x9d thermocouple. When the junction is heated a thermoelectric potential arises and can be measured across the unconnected ends. This is also known as the thermoelectric or Seebeck effect. This voltage is proportional to the temperature difference between the junction and the non-joined ends.
A conventional RF ablation catheter typically has a single tip electrode and a single temperature sensor mounted along the centerline of the tip electrode where temperature readings are not affected by the rotational orientation of the catheter. Although a temperature gradient typically exists in tip electrodes, wherein the electrode is hottest at the tissue interface and coolest on the opposite side which is in contact with circulating blood, the centerline sensor provides a moderate output by which it can be determined whether the temperature of the tissue contacted by the electrode is being raised sufficiently, and whether a therapeutic lesion is being generated.
In the case where a catheter has a band electrode, such as for the treatment of atrial fibrillation by the ablation of tissue, a single temperature sensor mounted to the band may not provide the temperature of the tissue contacting the band electrode. Typically the side of the band which is in direct contact with the tissue becomes significantly hotter than the rest of the band electrode that is cooled by the blood flow. Thus, the temperature reading can be dramatically influenced by the rotational orientation of the catheter during RF ablation. If the band is oriented so that the single temperature sensor is not in contact with the tissue during the application of ablation energy, not only would there be a time lag in the sensor reaching the tissue temperature, but due to the effect of the cooling blood flow, the sensor reading may never approach the actual tissue temperature.
To overcome the effect that the rotation orientation of the band electrode has on temperature sensing, two thermocouples, positioned at different locations of the band electrode, may be used. A theory is that having a sensor in contact with tissue is more likely. While attachment of multiple temperature sensors to the band electrode can result in a higher probability of sensing the actual tissue interface temperature, this also increases the number of wires occupying space within the catheter. As is well appreciated by those skilled in the art, an increase in the number of internal wires could mean an undesirable increase in catheter diameter to accommodate those wires. Conventional types of thermocouples each require a thermocouple wire pair. Two thermocouples at each band electrode would result in four wires per band electrode so that the use of multiple temperature sensors may not be practical, particularly where the catheter carries multiple band electrodes that require temperature monitoring.
The larger the catheter, the more traumatic it is to the patient. Also, the more difficult it may be to negotiate the patient""s vessels to position the catheter at the desired location in the heart. It is desirable to provide a catheter with as small a diameter as possible. A limiting factor in reducing the size of the catheter is the amount of devices and leads that must be carried inside the catheter. In the case of a catheter having ten band electrodes with two thermocouple temperature sensors at each electrode, a total of fifty wires would be necessary; one power wire for each electrode and two wires for each thermocouple. The size of fifty wires inside a catheter can be significant, causing an increased diameter of the catheter. Yet it is desirable to retain the electrodes and the associated temperature sensors so that more precise control over the energy applied to the biological tissue can be effected. Thus, it would be desirable to reduce the number of wires within a catheter, yet retain the same functionality.
Hence, those skilled in the art have recognized a need for a minimally invasive ablation apparatus that is capable of controlling the flow of current through a biological site so that lesions with controllable surface and depth characteristics may be produced and the ablation volume thereby controlled. Additionally, a need has been recognized for providing an electrode with multiple temperature sensors for providing reliable electrode/tissue interface temperature readings substantially independent of the rotational orientation of the catheter but with a reduced number of sensor leads. Similarly, a need has been recognized for a method for reliably determining the electrode/tissue interface temperature readings substantially independent of the rotational orientation of the catheter but with a reduced number of sensor leads The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to an apparatus and a method for controlling the application of energy to a biological site using a catheter having an energy application device, e. g., an electrode, and a sensor device e. g., a thermocouple, at its distal end for providing multiple temperature-sensitive locations on the electrode with a reduced number of leads.
In a first aspect, an apparatus includes a catheter having an electrode formed of a first metallic material. The electrode is disposed at a distal end of the catheter, the distal end adapted to be positioned so that the electrode is located proximal the biological tissue. The catheter also includes a first electrically conductive member formed of second metallic material, the first member is connected to the electrode at a first junction. Also included is a second electrically conductive member formed of a third metallic material; the second member is connected to the electrode at a second junction. The first, second and third metallic materials are chosen such that when the first and second junctions are at different temperatures a voltage output is produced across the electrode proportional to the temperature difference between the two junctions.
By selecting the first, second, and third metallic materials so that a voltage is produced which is proportional to the temperature average of the two points on the electrode, the present invention allows for the determination of the temperature at two distinct points on the electrode using only one pair of electrically conductive members. Thus the number of wires required to fit within a catheter is reduced, thereby allowing for a reduction in the catheter size.
In a detailed aspect of the invention, the first and second junctions are spaced apart on the electrode such that the voltage output is indicative of a temperature which is the average of the first and second junction temperatures. In a further detailed aspect, the first and second junctions are spaced apart on the electrode such that when the electrode is located proximal the biological tissue, one of the junctions is positioned near the electrode/tissue interface while the other junction is positioned in the biological fluid. In another detailed aspect, the electrode comprises a band electrode and the first and second junctions are located on the band electrode approximately 180 degrees apart around the band electrode inner circumference. In yet another detailed aspect, the second and third metallic materials are metallic materials having Seebeck coefficients relative to the first metallic material that are substantially equal in magnitude but opposite in sign.
In another detailed aspect of the invention, the apparatus further includes a power control system which is adapted to provide power for the electrode and to control the duty cycle of the power with the duty cycle having an on-period and an off-period within a duty cycle time frame. The power control system is further adapted to monitor voltage output produced across the electrode. In a further detailed aspect, the power control system controls the duty cycle of the power in response to the voltage output. In another detailed aspect, the catheter comprises a plurality of electrodes at its distal end, each electrode having a first and second electrically conductive member connected at a first and second junction and the power control system is further adapted to provide power to each of the electrodes wherein the power is selected such that at least two electrodes have voltage levels that differ from each other so that current flows between the two electrodes. In yet more detailed aspects, the power control system provides power with different phase angles to at least two of the electrodes; the power differs in phase by an amount greater than zero degrees but less than 180 degrees; and the power differs in phase by an amount approximately equal to 132 degrees.
In a further detailed aspect, the invention includes a backplate adapted to be positioned proximal the biological site so that the biological site is interposed between the electrodes and the backplate. The power control system is adapted to provide power to the electrodes wherein the power is selected such that at least one electrode has a voltage level that differs from the backplate so that current flows between at least one electrode and the backplate.
In yet another aspect, the invention is an apparatus for delivering energy to biological tissue located in a biological structure in which biological fluids flow past the tissue. The apparatus includes a catheter having a plurality of band electrodes formed of a first metallic material, the band electrodes disposed at a distal end of the catheter, the distal end is adapted to be positioned so that at least one of the band electrodes is located proximal the biological tissue. Also included is a plurality of first electrically conductive members formed of second metallic material, one first member is connected to one band electrode at a first junction. Further included is a plurality of second electrically conductive members formed of a third metallic material, one second member is connected to one band electrode at a second junction. The first, second and third metallic materials are chosen such that when the first and second junctions are at different temperatures a voltage output is produced across the electrode proportional to the temperature difference between the two junctions. Also included is a power control system adapted to provide power to each band electrode and to control the duty cycle of the power with the duty cycle having an on-period and an off-period within a duty cycle time frame. The power control system is further adapted to monitor voltage output produced across each electrode. Still further included is a backplate adapted to be positioned proximal the biological tissue so that the biological tissue is interposed between the electrodes and the backplate.
In a further aspect, a method for monitoring the temperature at the interface between an electrode and biological tissue during ablation of the biological tissue includes the step of positioning a catheter proximal the biological tissue to be ablated. The catheter has an electrode formed of a first metallic material and first and second electrically conductive members connected to the electrode at a first junction and a second junction, respectively. The first and second electrically conductive members are formed of second and third metallic materials, respectively, such that when the two junctions are at different temperatures, a voltage output is produced across the electrode proportional to the temperature average of the two junctions. The first and second electrically conductive members are spaced apart on the electrode. The method further includes the steps of positioning the electrode against the tissue for ablation so that a portion of the electrode is available for contact with the fluids in the biological structure and measuring the voltage output across the electrode as an indication of a temperature which is the average of the two junction temperatures.
In a detailed aspect, the method further includes the steps of placing the first junction in contact with the biological tissue and the second junction in contact with the biological fluid; measuring the temperature of the biological fluid; and determining the temperature of the first junction from the average temperature. In another detailed aspect of the invention, the electrode is a band electrode and the method further comprises the steps of placing the first junction in contact with the biological tissue and the second junction approximately 180xc2x0 away from the first unction around the band electrode circumference; measuring the temperature of the biological fluid; and determining the temperature of the first junction from the average temperature.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.